Artificial intervertebral disc placement system

ABSTRACT

An intervertebral disc placement system includes a multi part intervertebral disc, such as a mobile core disc, and an associated placement instrument. The placement instrument holds the disc securely with the disc endplates angled for ease of placement of the disc into an intervertebral disc space and quickly releases the implant within the disc space. The disc includes upper and lower plates having notches in inner surfaces for engagement of the placement instrument. The placement instrument has a rotatable key configured to fit into the first and second notches to grasp the disc from the interior and eliminate the need for an external grasping mechanism which could interfere with disc placement.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/504,033, filed Jul. 16, 2009, which claims priority to U.S.Provisional Application No. 61/018,441, filed Jul. 17, 2008, entitled“ARTIFICIAL INTERVERTEBRAL DISC PLACEMENT SYSTEM” the full disclosuresof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to medical devices and methods. Morespecifically, the present invention relates to intervertebral discprostheses.

Back pain takes an enormous toll on the health and productivity ofpeople around the world. According to the American Academy of OrthopedicSurgeons, approximately 80 percent of Americans will experience backpain at some time in their life. On any one day, it is estimated that 5%of the working population in America is disabled by back pain.

One common cause of back pain is injury, degeneration and/or dysfunctionof one or more intervertebral discs. Intervertebral discs are the softtissue structures located between each of the thirty-three vertebralbones that make up the vertebral (spinal) column. Essentially, the discsallow the vertebrae to move relative to one another. The vertebralcolumn and discs are vital anatomical structures, in that they form acentral axis that supports the head and torso, allow for movement of theback, and protect the spinal cord, which passes through the vertebrae inproximity to the discs.

Discs often become damaged due to wear and tear or acute injury. Forexample, discs may bulge (herniate), tear, rupture, degenerate or thelike. A bulging disc may press against the spinal cord or a nerveexiting the spinal cord, causing “radicular” pain (pain in one or moreextremities caused by impingement of a nerve root). Degeneration orother damage to a disc may cause a loss of “disc height,” meaning thatthe natural space between two vertebrae decreases. Decreased disc heightmay cause a disc to bulge, facet loads to increase, two vertebrae to rubtogether in an unnatural way and/or increased pressure on certain partsof the vertebrae and/or nerve roots, thus causing pain. In general,chronic and acute damage to intervertebral discs is a common source ofback related pain and loss of mobility.

When one or more damaged intervertebral discs cause a patient pain anddiscomfort, surgery is often required. Traditionally, surgicalprocedures for treating intervertebral discs have involved discectomy(partial or total removal of a disc), with or without fusion of the twovertebrae adjacent to the disc. Fusion of the two vertebrae is achievedby inserting bone graft material between the two vertebrae such that thetwo vertebrae and the graft material grow together. Oftentimes, pins,rods, screws, cages and/or the like are inserted between the vertebraeto act as support structures to hold the vertebrae and graft material inplace while they permanently fuse together. Although fusion often treatsthe back pain, it reduces the patient's ability to move, because theback cannot bend or twist at the fused area. In addition, fusionincreases stresses at adjacent levels of the spine, potentiallyaccelerating degeneration of the adjacent discs.

In an attempt to treat disc related pain without fusion, an alternativeapproach has been developed, in which a movable, implantable, artificialintervertebral disc (or “disc prosthesis”) is inserted between twovertebrae. A number of different artificial intervertebral discs arecurrently being developed. For example, U.S. Patent ApplicationPublication Nos. 2005-0021146, 2005-0021145, and 2006-0025862, which arehereby incorporated by reference in their entirety, describe artificialintervertebral discs. This type of intervertebral disc has upper andlower plates positioned against the vertebrae and a mobile corepositioned between the two plates to allow articulation, translation androtational motion between the vertebrae.

Another example of an intervertebral disc prostheses having a movablecore is the CHARITE artificial disc (provided by DePuy Spine, Inc.) anddescribed in U.S. Pat. No. 5,401,269. Other examples of intervertebraldisc prostheses include MOBIDISC (provided by LDR Medical), the BRYANCervical Disc (provided by Medtronic Sofamor Danek, Inc.), and thePRODISC (from Synthes Stratec, Inc.) and described in U.S. Pat. No.6,936,071. Some of these intervertebral discs are mobile core discswhile others have a ball and socket type two piece design. Althoughexisting disc prostheses provide advantages over traditional treatmentmethods, improvements are ongoing.

These known artificial intervertebral discs generally include upper andlower plates which locate against and engage the adjacent vertebralbodies, and a core for providing motion between the plates. The core maybe movable or fixed, metallic, ceramic or polymer and generally has atleast one convex outer articulation surface which mates with a concavearticulation recess on one of the plates in a fixed core device. In amovable core device two sets of articulation surfaces are provided. Inorder to implant these intervertebral discs, the natural disc is removedand the vertebrae are distracted or forced apart in order to fit theartificial disc in place. The plates may be inserted individually ortogether and with or without a core. It is desirable to reduce theduration of the artificial disc procedure by implanting the disc in anassembled configuration. However, when holding the disc for implantationit is desirable to prevent contact of the placement tool with the boneintegration surface of the disc to avoid damage to any bone integrationstructures or coatings thereon.

Currently available artificial intervertebral discs are held anddelivered with a variety of different instruments and techniques. Itwould be desirable to provide a disc system with a simple placementinstrument which easily and securely grasps the implant for insertion.

In addition, it would be desirable to hold the disc in the implantationinstrument in an articulated or angled insertion configuration toprevent the need for over distraction of the disc space.

Therefore, a need exists for an improved artificial intervertebral discplacement system which securely and easily holds the articulating platesof the disc in a fixed arrangement suitable for placement of the disc.

BRIEF SUMMARY OF THE INVENTION

According to the invention there is provided an artificial discplacement system including a placement instrument with a quick and easyassembly mechanism and quick deployment.

In accordance with one aspect of the invention, an intervertebral discplacement system includes an artificial disc and a placement instrument.The artificial disc comprises an upper plate having an upper vertebracontacting surface and a lower surface having a bearing surface thereon;a lower plate having a lower vertebra contacting surface and an uppersurface having a bearing surface thereon, wherein the upper and lowerplates are configured to articulate with respect to one another; and afirst notch in the lower surface of the upper plate and a second notchin the upper surface of the lower plate, wherein the first and secondnotches are aligned with one another. The placement instrument includesa handle and a rotatable key, wherein the key is configured to fit intothe first and second notches.

In accordance with another embodiment of the invention, anintervertebral disc placement system includes an upper plate having anupper vertebra contacting surface and a lower surface having a bearingsurface thereon; a lower plate having a lower vertebra contactingsurface and an upper surface arranged to articulate with respect to thebearing surface of the upper plate; a first notch in the lower surfaceof the upper plate and a second notch in the upper surface of the lowerplate, wherein the first and second notches are aligned with one anotherand face one another; and a placement instrument. The placementinstrument has a handle and at least one movable key and the at leastone key is configured to move into the first and second notches andretract toward the handle to secure the upper and lower plates to theplacement instrument.

In accordance with a further embodiment of the invention, anintervertebral disc placement system includes an artificial disc and aplacement instrument. The artificial disc comprises an upper platehaving an upper vertebra contacting surface, a lower surface having anarticulating surface thereon, anterior, posterior and two lateral edges;and a lower plate having a lower vertebra contacting surface, an upperarticulating surface arranged to articulate with respect to the upperplate, anterior, posterior and two lateral edges. The placementinstrument includes a handle and a disc receiving portion configured toreceive the upper and lower plates of the artificial disc in theplacement instrument in a locked configuration where the upper surfaceof the upper plate and the lower surface of the lower plate are angledsuch that the upper surface of the upper plate and the lower surface ofthe lower plate are closer together at an end of the disc remote fromthe placement instrument.

In accordance with an additional embodiment of the invention, an methodof inserting an artificial disc into a space between two adjacentvertebrae includes the steps of: positioning the artificial disc on aplacement instrument with upper and lower vertebrae contacting surfacesof the artificial disc positioned at an angle with respect to oneanother such that the vertebrae contacting surfaces are closer togetherat an end of the disc remote from the placement instrument; insertingthe artificial disc partway into the space under constraint to preventendplates of the prosthesis from articulating; releasing the prosthesisfrom constraint; and inserting the unconstrained prosthesis farther intothe space.

Other features of the invention are set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a bottom perspective view of a superior plate of an artificialdisc;

FIG. 2 is a top perspective view of the plate of FIG. 1;

FIG. 3 is a side cross sectional view of the superior plate taken alongline 3-3 of FIG. 1;

FIG. 4 is a top perspective view of an inferior plate of an artificialdisc;

FIG. 5 is a bottom perspective view of the plate of FIG. 2;

FIG. 6 is a side cross sectional view of the inferior plate taken alongline 6-6 of FIG. 4;

FIG. 7 is a perspective view of a core for use with the superior andinferior plates of FIGS. 1 and 4;

FIG. 8 is a perspective view of the assembled intervertebral disc heldby an placement instrument;

FIG. 9 is a perspective view of the placement instrument of FIG. 8;

FIG. 10 is a top view of the placement instrument of FIG. 9;

FIG. 11 is a side cross sectional view of the placement instrument takenalong line 11-11 of FIG. 10;

FIG. 12 is a top view of the placement instrument with a disc lockingkey in the unlocked position; and

FIG. 13 is a top view of the placement instrument with the key in thelocked position.

DETAILED DESCRIPTION OF THE INVENTION

An intervertebral disc placement system according to the presentinvention includes a multi part disc and an associated placementinstrument. The placement instrument holds the disc securely forplacement into an intervertebral disc space and quickly releases theimplant within the disc space.

FIGS. 1-3 illustrate a superior or upper plate 10 of an intervertebraldisc. FIGS. 4-6 illustrate a corresponding inferior or lower plate 20and FIG. 7 illustrates a core 30 positionable between the plates to forma complete articulating intervertebral disc. The upper and lower plates10, 20 include outer vertebral body contacting surfaces which areprovided with attachment enhancing features to ensure bone integration.The attachment enhancing features shown include one or more fins and anarray of serrations. In the embodiment shown the upper plate includestwo fins 12 and the lower plate includes one fin 22. The fins 12, 22 canbe an elongate fins pierced by one or more transverse holes 14 for boneingrowth. The fins 12, 22 can have one or both ends formed at an angle,such as a 45 degree angle to aid in insertion of the fins intocorresponding slots cut into the vertebral body. Particularly, theleading end of the fins 12, 22 should have an angled end surface.

The disc can be inserted laterally, from an anterior side, or from aposterior side. In the embodiment shown, the disc is designed forinsertion laterally into an intervertebral space from either side. Thelateral insertion of the disc is particularly useful in the lumbar spinewhere an anterior approach involves passing through the abdominal cavityand the posterior approach involve removal of a portion of the vertebra,such as the facets. Although the disc will be described herein asinserted laterally and is shown in a size appropriate for the lumbarspine, the system can be modified to accommodate other locations in thespine and other implantation approaches.

In the embodiment shown, two spaced apart superior fins 12 and a singlecentral inferior fin 22 are provided on the upper and lower plates,respectively, extending in a lateral direction. Alternatively, one, twoor more fins can be provided on either plate. In the example shown, thesingle fin is provided on one plate while the double fins are providedon the other plate to achieve a staggered arrangement particularlyuseful for multi-level disc implant procedures or in case an additionaldisc implant is required at a later time. The locations of the one andtwo fin plates can also be reversed so that the two fins are provided onthe inferior plate. The orientation of the fin(s) 12, 22 can also bemodified depending on the insertion direction for the intervertebraldisc.

The fins 12, 22 are configured to be placed in slots in the vertebralbodies. The transverse holes 14 may be formed in any shape and mayextend partially or all the way through the fins 12, 22. Preferably, thefins 12, 22 each have a height greater than a width and have a lengthgreater than the height. The fin length is preferably greater than onehalf of a dimension of the plate in the corresponding direction.

The fins 12, 22 provide improved attachment to the bone and preventrotation of the plates in the bone. In some embodiments, the fins 12, 22may extend from the surface of the plates 10, 20 at an angle other than90.degree. For example on one or more of the plates 10, 20 wheremultiple fins 12, 22 are attached to the surface the fins may be cantedaway from one another with the bases slightly closer together than theiredges at an angle such as about 80-88 degrees. The fins 12, 22 may haveany other suitable configuration including various numbers, angles,serrated edges, and curvatures, in various embodiments. In someembodiments, the fins 12, 22 may be omitted altogether.

The intervertebral body contacting surfaces of the plates 10, 20 alsocan include other geometries of bone integration structures includingserrations, teeth, grooves, ridges, pins, barbs or the like. When thebone integration structures are ridges, teeth, barbs or similarstructures, they may be angled to ease insertion and prevent migration.These bone integration structures can be used to precisely cut the boneduring implantation to cause bleeding bone and encourage boneintegration. Additionally, the outer surfaces of the plates 10, 12 maybe provided with a rough microfinish formed by blasting with aluminumoxide microparticles or the like to improve bone integration. In someembodiments, the outer surface may also be titanium plasma sprayed or HA(hydroxylapatite) coated to further enhance attachment of the outersurface to vertebral bone.

The intervertebral body contacting surfaces shown include a plurality ofserrations 16. The serrations 16 as shown are pyramid shaped serrationsextending in mutually orthogonal directions, however, other shapes mayalso be used. The serrations have a height of about 0.5-1 mm and a widthabout equal to their height. With passage of time, firm connectionbetween the plates 10, 20 and the vertebrae will be achieved as bonetissue grows over the serrated finish. Bone tissue growth will also takeplace about the fins 12, 22 and through the holes 14 therein, furtherenhancing the connection which is achieved.

Opposite the serrated vertebral body contacting surfaces of the plates10, 20, the plates have concave bearing surfaces 18, 28. These concavebearing surfaces are shaped to accommodate and retain a mobile core. Theparticular shape and curvature of the bearing surfaces 18, 28 can varyto accommodate different shaped cores or different applications.Surrounding the bearing surfaces 18, 28 are angled surfaces 19, 29 whichlimit the articulation of the disc. The angle at which the surfaces 19,29 are formed can be varied to form a disc with a selected maximumarticulation. The maximum articulation can be different in eachdirection including the anterior, posterior and lateral directions. Inone example, the disc is designed to allow angulations between the upperand lower plates of a maximum of .+−.12 degrees in the lateraldirection, .+−.12 degrees in the anterior/posterior direction, andunlimited rotation.

The core 30, as shown in FIG. 7, can be formed as a circular disc shapedmember with upper and lower bearing surfaces 32 which match thecurvature of the concave bearing surfaces 18, 28 of the plates. The core30 also has one or more annular rims 34 which cooperate with a retentionfeature 40 on at least one of the plates 10, 20 to retain the corebetween the plates when the intervertebral disc is implanted between thevertebrae of a patient. The core 30 is moveable with respect to both theupper and lower plates 10, 20 to allow articulation, translation androtation of the upper and lower plates with respect to one another. Thecore bearing surfaces 32 and concave bearing surfaces 18, 28 of theplates have the same radius of curvature which may vary depending on thesize of the intervertebral disc.

The retention feature 40 in the illustrated embodiment comprises aretention ring 42 on the lower plate 20, shown most clearly in FIG. 6.The retention ring 42 protrudes inwardly from an edge of the bearingsurface 28. Although a circumferential core retaining feature is shown,other core retaining features may also be used including at least thoseshown in U.S. Patent Publication Nos. 2005/0251262, 2005/0021146, and2005/0021145, which are incorporated herein by reference in theirentirety.

Although the core 30 has been shown as circular in cross section withspherically shaped bearing surfaces 32, other shapes may be usedincluding oval, elliptical, or kidney bean shaped. These non-circularshaped cores can be used to limit rotational motion between the upperand lower plates 10, 20. The bearing surfaces 18, 28, 32 of the platesand core are shown as spherical, however flat, cylindrical, tab andgroove, stepped or other shaped bearing surfaces may also be used.

Although the core 30 and plates 10, 20 have been shown as solid members,the core and plates may be made in multiple parts and/or of multiplematerials. The core can be made of low friction materials, such astitanium, titanium nitrides, other titanium based alloys, tantalum,nickel titanium alloys, stainless steel, cobalt chrome alloys, ceramics,or biologically compatible polymer materials including PEEK, UHMWPE, PLAor fiber reinforced polymers. High friction coating materials can alsobe used.

The present invention has been illustrated as a three piece articulatingdisc with a mobile core. The invention may also be embodied in a twopiece or ball and socket type disc which can be held by a placementinstrument in the same manner described below with respect to the threepiece disc.

The intervertebral disc according to the present invention providesarticulation in two directions as well as translation and rotation. Theplates 10, 20 are provided with notches 44 at one lateral end of eachplate for use in grasping the disc by a placement instrument 50 shown inFIG. 8. The placement instrument facilitates holding and manipulation ofthe disc for insertion or removal of the disc in an intervertebral discspace. The notches 44 allow the plates 10, 20 to be grasped and insertedsimultaneously in a locked orientation with or without a core 30 therebetween. The lateral edges of the plates 10, 20 also include a steppedsurface 46. On the end of the plates with the notches 22, the steppedsurfaces 46 engage a corresponding abutment surface on the placementinstrument to further secure the plates in the placement instrument.

The upper and lower plates 10, 20 may be formed from titanium, titaniumnitrides, other titanium based alloys, tantalum, nickel titanium alloys,stainless steel, cobalt chrome alloys, ceramics, or biologicallycompatible polymer materials including PEEK, UHMWPE, PLA or fiberreinforced polymers. The bearing surfaces or recesses 18, 28 areconcavely, spherically curved and can have a hard coating such as atitanium nitride finish. The plates 10, 20 may be treated with aluminumoxide blasting followed by a titanium plasma spray to improve boneintegration. Other materials and coatings can also be used such astitanium coated with titanium nitride, aluminum oxide blasting, HA(hydroxylapatite) coating, micro HA coating, and/or bone integrationpromoting coatings. Any other suitable metals or combinations of metalsmay be used as well as ceramic or polymer materials, and combinationsthereof to optimize imaging characteristics. Any suitable technique maybe used to couple materials together, such as snap fitting, slipfitting, lamination, interference fitting, use of adhesives, weldingand/or the like.

FIG. 8 illustrates the plates 10, 20 secured in the placement instrument50 and arranged with the upper plate at an angle with respect to thelower plate for placement of the disk. The angled placement orientationallows the plates 10, 20 to be inserted more easily into theintervertebral space with a narrow leading end formed by the angle ofthe plates. An angle between the upper vertebral body contacting surfaceof the upper plate 10 and the lower vertebral body contacting surface ofthe lower plate is at least 5 degrees. Preferably, the angle is about15-30 degrees, and more preferably about 25 degrees.

FIG. 9 illustrates the disc holding end of the placement instrument 50is mounted on a shaft 60. The instrument 50 includes a pair of arms 52having flat surfaces on opposite top and bottom sides for supporting theplates interior surfaces. An abutment surface 54 overhangs a groove 56which is configured to receive the protruding stepped surface at theends of the 46 of the plates. A rotatable key 58 functions as a lockingmechanism and is received in the notches 44 of the plates. Theengagement of the rotatable key 58 in the notches 44 allows the plates10, 20 to be made without any outwardly extending engagement features.This can provide an advantage of a lower profile over many of the knowdevices having exterior rims or grooves for engagement by a placementinstrument. The key engagement system of the placement instrument alsoprovides the advantage of a narrower placement profile than thoseplacement instruments which grasp the side surfaces of a disc.

FIGS. 12 and 13 show the operation of the handle end of the placementinstrument 50 which includes the handle 62, a rotatable knob 64, and amovable lever 66. The movable lever 66 is connected through a centralshaft 68 of the handle (shown in FIG. 11) to the key 58 and functions asa key rotating mechanism to move the key from an unlocked position shownin FIG. 12 to a locked position shown in FIG. 13. The rotatable knob 64functions as a retraction mechanism to retract the key 58 toward thehandle 62 to secure the plates against the abutment surfaces 54. Inoperation, the plates 10, 20 are place in the disc holding end of theplacement instrument with the notches 44 aligned over the key 58 and thekey in the unlocked position of FIG. 12. The key 58 is then rotated 90degrees by moving the lever 66 90 degrees causing the ends of the key toenter the notches 44. The plates 10, 20 are then pulled tight againstthe abutment surfaces 54 of the instrument by rotating the knob 64 toretract the key toward the handle. This pulls the key 58 against theside of the notches 44 to lock the plates tightly in place.

The disc holding end of the placement instrument 50 can be marked, suchas by markers 70 which include either two dots or one dot indicatingthat the position for either a two fin or one fin plate. Other markingor indicia can also be used to identify the correct location and/ororientation of the plates, such as labeling (superior, inferior), colorcoding, or other insignia.

The artificial intervertebral disc is surgically implanted betweenadjacent spinal vertebrae in place of a damaged disc. Those skilled inthe art will understand that the damaged disc is partially or totallyremoved according to known procedures and the adjacent vertebrae areforcibly separated from one another to provide the necessary space forinsertion of the disc. The plates 10, 20 are slipped into place betweenthe vertebrae with their fins 12, 22 entering slots cut in the opposingvertebral surfaces to receive them. The plates 10, 20 may be insertedsimultaneously with or without the core 30. The disc may be inserted allthe way to a final resting place with the placement instrument 50, orafter partial insertion of the disc, the individual plates 10, 20 can befurther advanced independently or together to a final position. Theinsertion of the disc partway into the intervertebral space whileconnected to the placement instrument and then the further insertion ofthe plates while they are free to move with respect to one anotherallows the disc to moved to a final position while it is free to take onthe angulation or lordosis of the space. This reduces the problem ofoverdistraction of the disc space.

Once the disc has been partially or completely inserted, the placementinstrument 50 is removed from the plates 10, 20 by simply rotating thelever 66 90 degrees back to the unlocked position of the key 58. Oncethe disc has been inserted, the vertebra move together to hold theassembled disc in place.

The vertebral contacting surfaces of the plates 10, 20 including theserrations 16 locate against the opposing vertebrae and, with passage oftime, firm connection between the plates and the vertebrae will beachieved as bone tissue grows over the serrated finish. Bone tissuegrowth will also take place about the fins 12, 22 and through the holes14 therein, further enhancing the connection which is achieved.

While the exemplary embodiments have been described in some detail, byway of example and for clarity of understanding, those of skill in theart will recognize that a variety of modifications, adaptations, andchanges may be employed. Hence, the scope of the present inventionshould be limited solely by the appended claims.

What is claimed is:
 1. A method of inserting an artificial disc into aspace between two adjacent vertebrae, the method comprising: positioningthe artificial disc on a placement instrument with upper and lowervertebrae contacting surfaces of the artificial disc positioned at anangle with respect to one another such that the vertebrae contactingsurfaces are closer together at an end of the disc remote from theplacement instrument; rotating a key of the placement instrument tosecure the artificial disc on the placement instrument by receiving endsof the key in notches on inner surfaces of the artificial disc;inserting the artificial disc partway into the space under constraint toprevent endplates of the prosthesis from articulating; releasing theprosthesis from constraint; and inserting the unconstrained prosthesisfarther into the space.
 2. The method of claim 1, wherein the upper andlower vertebrae contacting surfaces are angled at an angle of at least15 degrees when the artificial disc is positioned on the placementinstrument.
 3. The method of claim 1, wherein a disc receiving portionof the placement instrument is configured to receive a lateral edge ofthe artificial disc and the steps of inserting the artificial discinclude placement of the disc from a lateral aspect of a patient'sspine.
 4. The method of claim 1, wherein the upper plate is held by theplacement instrument at an angle with respect to the lower plate forplacement of the disk.
 5. The method of claim 4, wherein the artificialdisc is inserted into the space from a lateral aspect of a patient'sspine.
 6. A method of inserting an artificial disc into a space betweentwo adjacent vertebrae, the method comprising: providing an artificialdisc comprising: an upper plate having an upper vertebra contactingsurface and a lower surface having a bearing surface thereon; a lowerplate having a lower vertebra contacting surface and an upper surfacehaving a bearing surface thereon, wherein the upper and lower plates areconfigured to articulate with respect to one another; and a first notchin the lower surface of the upper plate and a second notch in the uppersurface of the lower plate, wherein the first and second notches arealigned with one another; and positioning the upper and lower plates ofthe artificial disc on a placement instrument; rotating a key of theplacement instrument to secure the artificial disc on the placementinstrument by receiving ends of the key in the first and second notchesto constrain motion of both the upper and lower plates; inserting theartificial disc into the space; and releasing the prosthesis fromplacement instrument.
 7. The method of claim 6, wherein the placementinstrument key has a locked position and an unlocked position, andwherein in the locked position the key is positioned with ends of thekey in the first and second notches.
 8. The method of claim 6, whereinthe placement instrument includes first and second abutment surfacesconfigured to contact the upper surface of the upper plate and the lowersurface of the lower plate.
 9. The method of claim 6, wherein theplacement instrument includes a disc holding end and a handle end, andwherein the key is an elongated member disposed at the holding end androtatable by a key rotating mechanism at the handle end of the placementinstrument.